1. Field of the Invention
The present invention relates to a valve timing control apparatus.
2. Description of Related Art
Conventionally, a fluid-drive valve timing control apparatus is known, and has a housing rotatable with a crankshaft and a vane rotor rotatable with a camshaft. A valve timing is controlled by working fluid supplied from a supply source, synchronized with a rotation of an engine. The control apparatus controls working fluid to flow into or out of operation chambers partitioned by vanes of the vane rotor in a rotating direction inside of the housing, thereby changing a rotation phase of the vane rotor relative to the housing.
A variation torque is applied to the vane rotor from the camshaft so as to bias the vane rotor alternately between an advance side and a retard side with respect to the housing. When the rotation phase of the vane rotor is changed with respect to the housing, the variation torque is applied to the advance side or the retard side. At this time, a volume of the operation chamber is instantaneously enlarged. If introduction of working fluid becomes late relative to the operation chamber, inside pressure of the enlarged chamber becomes negative, that is, becomes lower than atmospheric pressure, so that air outside of the apparatus is drawn into the enlarged chamber through a clearance. The drawn air has foam or bubble state in the chamber, and is mixed into working fluid in the chamber. Coefficient of elasticity of the mixed air foam is small in the chamber, so that the vane rotor may have abnormal movement by elastic reaction force generated to the variation torque. In this case, accurate controlling of the valve timing becomes difficult.
JP-A-2000-345869 (U.S. Pat. No. 6,505,585) describes a valve timing control apparatus, that compulsorily changes a rotation phase into an advance side and a retard side with respect to the housing. That is, working fluid is controlled to flow into or out of a chamber so as to discharge air foam from the chamber. The compulsory change of the rotation phase is performed if a rotation speed of an engine exceeds a predetermined speed in JP-A-2000-345869.
However, even when the rotation speed of the engine exceeds the predetermined speed, the chamber may not always have the air foam, so that the compulsory change of the rotation phase may be performed in vain. Specifically, while the engine continues to have high rotation, a pressure of working fluid supplied from a supply source is high in synchronization with the engine rotation. In this case, introduction of working fluid is not late, so that the mixing of air foam is not generated.
That is, even if the engine instantaneously has a low rotation in such high rotation state, the compulsory change of the rotation phase is unnecessary, because the mixing of air foam is not generated. Rather, the compulsory change of the rotation phase may cause a rapid change in operation state of the engine. Therefore, it is desirable to perform the compulsory change of the rotation phase pinpointly at a timing necessary for preventing the abnormal movement of the vane rotor.